The present invention is directed to a method of surface treating workpieces to be coated where the workpieces are formed of a cement, bituminous or other bonded material, such as in roads, airports and bridge surfaces and the like where the surface to be coated is roughened and cleaned before applying the coating or cover layer.
At the present time various methods are used for roughening the surface, for instance, sandblasting, high pressure water jets, needle hammer treatment, countersinking or milling by appropriate diamond or hard metal tipped tool bits, brushing with steel wire brushes, and other similar procedures. In addition to the above mechanical methods, chemical treating methods are also known for roughening and partially removing the surface of structural components and workpieces to be coated. Adhesive agents are also used for increasing the adhesion of the layers to be applied, such as to increase the adhesive bond between old and new concrete. For the most part, a substance constituted of synthetic material or other materials (for instance cement mortar) is applied in the form of a thin layer on a more or less roughened surface of the old concrete. Such substances and methods are offered by many manufacturers and demonstrate how the scientific investigations of Hilsdorf and Belli (influence of adhesive bridges upon the durability of repairs with cement mortar: in Research Road-building and Process Technology, Vol. 342, 8 47-89, 1981), have not resulted in the desired effects, rather they tend to in some cases to be increasingly prone to cracking and weakening of the bond.
The characterization of the adhesive properties of layers or coatings were in the past mostly performed by the pull-off test. A cylinder-shaped test specimen is drilled out perpendicularly to the bonding face extending below the material bond. A steel plate of the same cross-section is bonded to the end face of the test specimen and the drill core is pulled off in the axial direction by a tension testing device. Since the adhesive strength is frequently weaker than the strength of the basic material, a crack formation or fracture occurs more or less in the bonding face. The maximum force required is measured, and divided by the area of the cross-section and the adhesive tensile strength is determined as the sole measured magnitude. This circumstance is viewed as a particular disadvantage of this method, since based on the measured results, it cannot be judged whether the bonding separation occurs in the form of a "brittle" or "ductile" fracture or whether little energy (brittle fracture) or a considerable amount of energy (ductile fracture) has to be expended for breaking the bond. Accordingly, the pull-off test is an inadequate method of characterizing the adhesive bond of material. In spite of this fact, this method has been embodied in several standards.
This situation was improved by the testing device and associated test specimen shapes described in AT 390328. This testing device is suitable for determining fracture mechanism characteristic values of material and material bonds. This method eliminates the above-mentioned disadvantage of the pull-off method. The test method involves basically the wedge splitting arrangement. The test specimen is split at a stable crack-progression by a loaded wedge device on dice or cylinder-shaped test specimens provided with a groove and a stress raiser (position in the material bond). During the measurement the load displacement curve (splitting force as a function of the force displacement or crack in the notch opening) is determined, this affords all the data for complete characterization of the fracture behavior of the material or of the material bond. The surface under the load displacement curve represents the fracture energy required for complete severance of the test specimen. If the fracture energy is divided by the size of the fracture area (only the projection of the ligament area is used), the specific fracture energy G.sub.f is obtained. The G.sub.f value is a characteristic value of the material and represents a measure of resistance to crack propagation. Small G.sub.f values point to a "brittle" material severance and high G.sub.f values indicate "ductile" material severance. It is possible to differentiate between a brittle and ductile material severance on the basis of such a test. Additional information can be gathered directly from the load displacement diagram of the maximum value of the force (F.sub.max -value). The "notch tensile strength" can be computed from this value. This value is to be viewed in a certain interrelationship with the adhesive strength (determined by the pull-off test).
The characterization of adhesive bonds by this new testing method brings with it new knowledge having decisive significance and considerable influence on the shaping and design of material bonds.
In the publication "Adhesive Power Measurements of Bonds Between Old and New Concrete" in The Journal of Materials Science, 26 (1991), pages 5189-5194 of E. K. Tschegg and S. E. Stanzi, the influence of different old concrete surface treatments as well as bonding agents upon the adhesive old-new concrete bond is investigated by means of the new wedge-splitting method. If these measurement results for the specific fracture energy and for the F.sub.max values of the different tested types of bond are standardized to the values of homogeneous concrete then we obtain the following information:
______________________________________ Pretreatment of the old concrete surface or G.sub.f /G.sub.f0 F.sub.max /F.sub.0max adhesive agent % % ______________________________________ Homogeneous concrete 100 100 Shell smooth 9 34 Sand blasted 20 55 Treated with needle hammer 17 50 with emulsion 7 29 ______________________________________
These values are determined on the following old or new concrete materials:
Old concrete: approximately 1/2 years old, average quality (B400-500), largest grain size 16 mm PA0 New concrete: 28 days old, average quality (B400), largest grain size 16 mm
From the above table it is established that the standardized F.sub.max value of most investigated specimens actually approaches by about 50% of the value of the homogeneous concrete. This value, however, is not governed or decisive for crack formation bonds, rather it is the specific fracture energy G.sub.f. Here the normal standard G.sub.f values are at approximately 10 to 20% (referred to homogeneous concrete). Therefore, if the results of the tear-off tests (similar to the F.sub.max values) are used in judging the mechanical properties of the adhesive bond, then with a pretreatment by "sandblasting" a value is obtained of approximately 50%, thus approximately half the old concrete values on the contrary. The material characteristics G.sub.f, which is a measure of the crack resistance of the bond and, therefore, has a much higher significance, is more significant for concrete construction and results from this pretreatment in a value of approximately 20%, that is, approximately 1/5 of the old concrete G.sub.f value. As a result, it can be seen from this example that formerly bonds of cement or bituminous bonded material were completely wrongly judged by the tear-off tests and, therefore, the development of measures for improving the adhesion is not attempted or no additional increase in adhesive strength was expected.
In addition, it follows from the above table that only a relatively small increase in adhesion as compared to no treatment at all ("Shellsmooth") has been achieved by the generally used pretreatment of old concrete surfaces by methods such as sandblasting.